Alkaline composition



ALKALINE COMPOSITION Frank P. Ilenda and Howard E. Everson, Painesville,

Ohio, assignors to Diamond Alkali Company, leveland, Ohio, a corporation of Delaware No Drawing. Application September 28, 1954 Serial No. 458,960

4 Claims. (Cl. 252-156) Various fused alkaline salt compositions have heretofore found wide application in metal treating operations, including carburizing, normalizing, martempering, austempering, stress relieving, quenching, hardening, annealing, and metal cleaning. This invention is particularly directed to improvements in fused salt metal cleaning baths and contemplates metal cleaning baths as including not only descaling baths, which are used to condition and remove surface oxide coatings and other surface imperfections, but also de-investing baths employed to remove siliceous material and other types of mold-forming-substances adhering to cast metal objects.

Investment casting, employing molds formed of siliceous or other heat-resistant substances, such as various zirconia compounds, is highly advantageous in the formation of intricately shaped parts to relatively close tolerances. Although a number of separating agents .or mold washes have been proposed and are used both in precision casting and in other casting operations to facilitate separation of the cast article from the mold, such materials frequently do not satisfactorily remove or de-invest the casting.

It is, therefore, a principal object of the present invention to provide new and improved compositions of matter which are useful in metal-treating operations generally and are particularly advantageous in descaling and de-investing processes.

A further object of the invention is the provision of new and improved methods for descaling and tie-investing metals and alloys.

These and other objects and advantages of the invention will appear more fully from the following descriptron.

It has heretofore been proposed to employalkaline salt baths in the removal of oxides from steel surfaces. Moreover, alkaline salt baths have been used in prior deinvesting operations. Typical of such prior art alkaline salt baths has been a fused salt consisting essentially of an alkali metal hydroxide, which admittedly possesses certain advantageous properties as a metal cleaning agent. However, such prior art fused alkaline salt baths have not satisfactorily solved the problem of de-investing castings, particularly intricately shaped cast articles, and, in many instances, have not been adequate in metal cleaning and descaling applications.

The present invention avoids the difliculties heretofore encountered and obtains highly advantageous results in the treatment of metal surfaces generally by employing as a treating material a composite salt bath consisting essentially of, on a weight basis, a major proportion of at least one compound selected from the class consisting of alkali metal hydroxides and a minor proportion of a source of fluoride ion. tive in the removal of investment materials from metal. Typical of such investment materials are zirconia compounds, such as zirconia silicate; clays, aluminum oxide, magnesium oxide, and various siliceous materials, the

. expression siliceous materials, as used herein, being in-,

atent' O Such baths are particularly eiiectended to include various silica-containing substances commonly employed in the preparation of investment molds, such as sand, silica powder, silica flour, sodium silicate, and the like.

More particularly, a composite salt of this invention consists essentially of, on a weight basis, a preponderant amount of at least one alkali metal hydroxide, i.e., a hydroxide of sodium, potassium, cesium, rubidium, and lithium, sodium preferred, and a minor proportion of an alkali metal fluoride, such as sodium fluoride, potassium fluoride, and the like, and/or an alkaline earth metal fluoride, such as calcium fluoride, strontium fluoride, or barium fluoride. In many instances, superior results are obtained when the major proportion of the salt comprises a mixture of potassium hydroxide and sodium hydroxide, a preponderant amount of the latter being preferred.

The practice of this invention contemplates treating metal articles by immersion in, or otherwise contacting with, the above-described alkaline salts in a fused condition. It will be understood, of course, that the temperature to which the salt should be heated will be dictated by a number of factors, including the particular bath formulation, the material being treated, the equipment used, and whether it is desired to combine certain heattreating operations with the metal cleaning process. Satisfactory results have been obtained, however, using such alkaline baths in the fused condition at temperatures within the range from approximately 600 to 1100" F. In descaling treatments, particularly descaling of stainless steels, temperatures within the range from about 900 to 950 F. are preferred. in de-investing cast carbon steel articles, temperatures of 1100" F. or higher are satisfactory, although many types of castings may be deiuvested at lower temperatures, such as temperatures within the range from 900 to 950 F.

V The time of immersion or contact with the fused salt will depend on the bath temperature and other factors. In general, the time may range from a few seconds, for exampleflS to 30 seconds, to a number of hours, such as 6 or 8, or higher, depending on the particular application.

In order that those skilled in the art may more completely understand the present invention, the following specific examples of alkaline salt baths within the scope of this invention are offered wherein the quantities are expressed in parts by weight:

Example I NaOH KOl-l 15 NaF 5 Example II NaOH 5S KOH I 35 NaF 10 Example III 7 NaOH NaF 10 Example IV NaOH 80 NaF 20 Example V KOH 90 NaF 10 Example VI NaOH 80 KOH 10 NaF 10 Example VII In general, superior results are obtained using an alkaline salt bath containing, on a weight basis, from about 50% to 95% of sodium hydroxide, 15% to 50% of potassium hydroxide, and 4% to 15% of an alkali metal fluoride or an alkaline earth metal fluoride, the preferred fluoride at present being sodium fluoride. In certain cases, larger amounts of the alkali metal fluoride and/or alkaline earth metal fluoride may be used. Generally, however, the maximum practicab e amount of fluoride varies and is dictated by its solubility in the fused salt at the operating temperatures employed. For example, in a salt bath consisting of NaOH and KOH in a 6:1 ratio, the solubility of NaF therein varies from 4.3% by weight at 650 F. to 16.5% at 1050" F.

The practice of this invention, while applicable to a wide variety of both ferrous and non-ferrous metals and alloys, including such metals as titanium, is particularly advantageous in the treatment of stainless steels, such as chromium-iron alloys, nickel-iron alloys, and other chromium-nickel-iron alloys which resist descaling and deinvesting by prior art fused alkaline salts. As used herein, the term metal is intended to include both metals and alloys, as well as metallic mixtures.

In the practice of this invention, it is understood, of course, that the precise process details may be varied, depending upon the particular metal or alloy being treated, as well as the characteristics of the surface coating, if any, to be removed and/or conditioned. However, in most descaling operations. it is advantageous to employ a method which includesthe successive steps of immersing the metal in a molten salt bath of the invention for a specified period of time; quenching'the thus'treated metal in water; immersing the quenched metal in a dilute inorganic acid, such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, or hydrofluoric acid, with or without corrosion inhibitor, while the acid is maintained at an elevated temperature, typically within the range from about 150 to 180 F.; rinsing the acid-treated metal in water; and finally immersing the rinsed'metal In either dilute nitric acid or 'a dilute mixture of nitric acid and hydrofluoric acid, either of such acids or acid mixtures being maintained at an elevated tem erature, typically within the range from 140 to 180 F. Such a nitric acid or nitric'ac'id-containing solution treatment serves to brighten. and passivate the metal surface and 18 particularly advantageous in the treatment of stain- .less steels. Following such passivation, the metal preferrablyilsrinsed inllot Water.

As in the case of descaling, the particular procedure to be followed in de-investing castings depends upon the type of metal to be de-invested and also the characteristics and composition of the investment material adhering to the casting. However, the following successive combination of steps constitutes the presently preferred method. The metal to be de-invested is immersed in a molten salt bath embodying the invention at a desired temperature and for a specified period oftime. In many instances, only a water-quench in addition to the salt immersion step is necessary. Typical of such alloys which may be so treated are stainless steel and various cobalt-base alloys. In other cases, it is advantageous to use the same process in de-investing as is herein suggested for descaling operations.

As indicated by the foregoing paragraphs, it is fre quently desirable, after the fused salt treatment, to contact the surface of the metal being treated with an inorganic acid at an elevated temperature. It will be understood, of course, that the conditions of acid treatment may be varied. In general, however, excellent results are obtained in providing a clean bright metal surface by using .relatively dilute inorganic acids, i.e., about 5% to 20% by weightaqueous solution, preferably at elevated temperatures within the range from about 125 to 200 F., and for times ranging from a few seconds, i.e., 5 to 10 seconds, to a more extended period, in certain instances as high as 30 minutes or more.

Example XIII To illustrate the effectiveness of salt compositions of the invention as de-investing agents, a precision cast turbine blade of Stellite 21, i.e., a cobalt base alloy having the following composition: 0.25% C, 27.0% Cr, 2.0% Fe, 5.5% M0, 2.5% Ni, and the balance Co, is immersed in an alkaline salt bath comprising by weight sodium hydroxide, 15 by weight potassium hydroxide, and 5% by weight sodium fluoride at 950 F. for 5 minutes. Upon removal from the bath and after a water rinse, the investment material is completely removed from the casting, including the investment material originally present in a blind hole in the turbine blade.

Example XIV Example XV A fused bath is prepared by heating together 77.5 parts by weight of sodium hydroxide, 13.5 parts by weight of potassium hydroxide, and 9 parts by weight of sodium fluoride. Into this bath, maintained at a temperature of 1100 F., are introduced 6 steel alloy castings which have been formed in investment'molds and bear upon their surfaces a substantial quantity of zirconia-silicate investment material. After 30 minutes immersion, the castings are removed from the bath and are completely de-invested.

Example XVI To illustrate effectiveness in descaling operations of compositions embodying the invention, samples of stainless steel (S.A.E. 51410) bearing a substantial quantity of surface oxide scale are immersed in a fused alkaline salt bath comprising 80% by weight sodium hydroxide, 10% by weight potassium hydroxide, and 10% by weight soditum fluoride for 5 minutes at a temperature .of 925 F.

Example XVII Samples of stainless steel (S.A.E. 30310) bearing a surface oxide coating are immersed in a fused alkaline salt bath comprising 80% by weight sodium hydroxide, 15% by weight potassium hydroxide, and by weight potassium fluoride for 5 minutes at 925 F. The thus treated samples are then quenched in water and contacted with a solution of sulfuric acid at 170 F. for 2 minutes.

The samples are then rinsed in water andbrightened by immersion: in an aqueous solution comprising 10% by weight nitric acid and 2% by weight hydrofluoric acid at 140 F. for 2 minutes. The samples are then watern'nsed. It is noted that the scale is completely removed and the resultant su'rfaceis clean and bright.

The foregoing description of the present invention has been given for illustrative purposes and no undue limitations should be deduced therefrom. Accordingly, it is to be understood that variations and modifications capable of being accomplished by the mere exercise of skill of the art are to be embraced within the scope of the appended claims.

What is claimed is:

1. A composite alkaline composition consisting essentially of, on a weight basis, -95% sodium hydroxide, 15-50% potassium hydroxide, and 4-15% of an alkali metal fluoride.

2. The composition of claim 1 wherein the alkali metal fluoride is sodium fluoride.

3. A composition consisting essentially of, on aweight basis, sodium hydroxide, 15% potassium hydroxide, and 5% sodium fluoride.

4. A composition consisting essentially of, on a weight basis, 77.5% sodium hydroxide, 13.5% droxide, and 9.0% sodium fluoride.

potassium hy- References Cited in thefile of this patent UNITED STATES PATENTS 1,111,444 Dunton Sept. 22, 19 14 1,705,944 Siegmund Mar. 9, 1929 2,062,273 Reed Nov. 24, 1936 2,241,984 Cooper May 13, 1941 2,395,694 Spence Feb. 26, 1946 2,468,006 Webster Apr. 19, 1949 2,569,158 Francis Sept. 25, 1951 2,654,701 Calderon et al Oct. 6, 1953 2,801,215 Webster July 30, 1957 FOREIGN PATENTS 598,466 Great Britain Feb. 19, 1948 

1. A COMPOSITE ALKALINE COMPOSITION CONSISTING ESSENTIALLY OF, ON A WEIGHT BASIS, 50-95% SODIUM HYDROXIDE, 15-50% POTASSIUM HYDROXIDE, AND 4-15% OF AN ALKALI METAL FLUORIDE. 